python - Qt Dynamic Properties - Works for widget if not parented only?

Question

This question already has answers here: question from:https://stackoverflow.com/questions/65939834/qt-dynamic-properties-works-for-widget-if-not-parented-only

Answer 1

The standalone widget works because Qt uses the basic stylesheet properties (background and border) only for top level windows, assuming that the top level window is of the same class as that specified in the stylesheet.

When a QWidget has a parent, it does not paint anything on its own.
As the documentation explains:

If you subclass from QWidget, you need to provide a paintEvent for your custom QWidget

So, in PyQt/PySide terms:

class Example(QtWidgets.QWidget):
    # ...
    def paintEvent(self, event):
        opt = QtWidgets.QStyleOption()
        opt.initFrom(self)
        qp = QtGui.QPainter(self)
        self.style().drawPrimitive(
            QtWidgets.QStyle.PE_Widget, opt, qp, self)